1. Vacuuming the PS2 from the outside, or
2. (WARNING: voiding warranty) Cleaning the PS2 internally:
(a) Lay the PS2 on its back. Remove the screw covers (black square things), then remove the screws - remember to take notes of which screw goes where since there are some longer screws.
(b) Lay the PS2 on its feet (horizontally). Try to remove the PS2 cover by ziggling, tilting (back-side, up), and pulling it up. Watch out for the cable connecting the Reset and Eject buttons.
(c) With the cover removed, unscrew the CD/DVD drive's cover using a screwdriver with small tip (jewel-screwdriver?).
(d) With the lens visible, clean it however you like. Personally, I used a Q-tip with some alcohol solution and cleaned the lens using circular motion; then I dryed it off with a dry Q-tip. While inside, you might want to vacuum off the dust and clean the fan, etc.